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MYHM (previously called IMM) is a muscle disease found in Quarter Horses

Full name

Myosin-heavy chain myopathy

Alternate names

Immune mediated myositis (IMM)

Breeds known to be affected

Quarter Horses and related breeds

Inheritance pattern

Autosomal codominant


MYHM can cause two different types of disease. One is muscle weakness and stiffness, followed by a rapid, significant loss of muscle, particularly from the topline of the horse. This is often associated with an infection or vaccination, particularly with Strangles or other respiratory viruses. This syndrome was originally named “immune mediated myositis” or IMM. A mutation associated with IMM was identified by researchers at UC Davis and Michigan State University in the Myosin Heavy Chain 1 (MYH1) gene.

It was later found that horses carrying this mutation were also susceptible to non-exertional rhabdomyolysis. This manifests as stiffness, firm muscles, short stride and often is accompanied by dark coloured urine. Unlike many other forms of tying up, it is not associated with exercise. The dark urine is an indicator that muscle damage has occurred. The identification of this second set of clinical signs associated with the mutation caused IMM to be renamed Myosin-heavy chain myopathy.

MYHM is co-dominant, meaning that the action of the variant is independent of the second variant. If a horse has one copy of the MYHM mutation it can be affected with MYHM. MYHM has ‘incomplete penetrance’ – so not every horse carrying this mutation will show the same severity of symptoms. It is thought that if a horse has two copies it is more likely to be more severely affected.

Interpretation of results

n/n: Horse does not carry the mutation associated with MYHM.

MYHM/n: Horse has one copy of the mutation associated with MYHM.

MYHM/MYHM: Horse has two copies of the mutation associated with MYHM. It will pass MYHM on to all its offspring and is likely to be more severely affected than a horse with only one copy.


Finno et al. A missense mutation in MYH1 is associated with susceptibility to Immune-mediated myositis in Quarter Horses. Skeletal Muscle 2018; 8:1 doi: 10.1186/s13395-018-0155-0

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